Pulse radar transmitting oscillator

ABSTRACT

An oscillator circuit for use in a pulsed radar transmitter includes a reference oscillator of one frequency of, a voltage controlled oscillator controlled to transmit a radio frequency at a different frequency, and a switch for connecting the two oscillators together to inject lock the voltage controlled oscillator to the frequency of the reference oscillator when no transmitting is occurring.

In the prior art, a pulsed radar system has utilized a magnetron as theradio frequency (RF) pulse producing element. The magnetron responds toa high voltage bias pulse by producing a high power transmitting signalat some preselected frequency and ideally either produces that frequencywithout changes in frequency as the magnetron is turned on and off or,when no bias pulse is present, produces no frequency. Magnetrons comeclose to the ideal. They are, however, bulky, heavy and costly whencompared to modern solid-state technology. It is therefore desirable toutilize a solid-state transmitting oscillator circuit. Such a circuitshould, like the magnetron, produce only the desired frequency whenpulsed and no frequency or at least not the preselected frequency whenturned "off." Such solid-state type oscillators undesirably exhibit widefrequency excursions upon turn-on. If the oscillator remains oncontinuously, signals produced by the oscillator tend to mask very weakradar return signals in a radar system utilizing the solid-stateoscillator thus reducing substantially the sensitivity of the system.

In accordance with the present invention, a first reference oscillatorproduces a frequency f₁. A second oscillator normally produces adifferent frequency f₂ but is responsive to the application of f₁ fromthe first reference oscillator for producing frequency f₁. Atransmitting means is responsive to frequency f₂ for transmitting f₂.Means responsive to a source of spaced-apart pulses connects the firstoscillator to the second oscillator only when said spaced apart pulsesare not present.

In the drawing:

FIG. 1 is one embodiment of the transmitting and receiving portion of apulsed radar system in block diagram form including the transmittingoscillator circuitry;

FIG. 2 is a second embodiment of the transmitting and receiving portionof a pulsed radar system in block diagram form including the transmitteroscillator circuitry; and

FIG. 3 is a set of waveforms useful in understanding the operation ofthe FIGS. 1 and 2 circuits.

Refer to FIG. 1 where encircled letters at various points in the figurecorrespond to similarly legended waveforms in FIG. 3. FIG. 1 is a blockdiagram of a pulse radar transmitter and receiver 10 such as utilizedfor airborne weather radar systems. A reference oscillator 12 such as atransferred electron oscillator produces a nominal frequency f₁ such as,for example, 9342 MHz. The oscillator is subject to slight variations infrequency which are compensated by the system as will be describedhereinafter. Oscillator 12 is coupled to a circulator 14 which iscoupled to terminal 16a of an electronic single pole double throw switch16. A second terminal 16b of switch 16 is coupled to a radio frequency(RF) power amplifier 18. Amplifier 18 is coupled to a circulator 20sometimes referred to as a duplexer. Circulator 20 is coupled to atransmit and receive antenna 22 of conventional design for transmittingRF pulses into the atmosphere and receiving reference return signalsthereform.

RF signals returned to antenna 22 are passed by circulator 20 to atransmit/receive limiter 24 coupled to circulator 20. Limiter 24 iscoupled to an RF amplifier 26. The purpose of limiter 24 is to preventhigh powered signals produced by amplifier 18 and leaked via path 20a ofcirculator 20 from damaging amplifier 26. Amplifier 26 is coupled to aninput terminal 30a of a frequency substractive mixer or a low pass mixer30. Mixer 30 is typically an image reject or double balanced mixer.

Circulator 14 at port 14a is coupled to the second input terminal 30b ofmixer 30 for passing signals present at terminal 16a to the mixer. Mixer30 produces at terminal 30c signals which are the difference infrequency between those applied to the mixer at terminal 30a andterminal 30b. That is, mixer 30 produces signals which are termedintermediate frequencies (IF). Terminal 30c is coupled to anintermediate frequency amplifier 32 which is in turn coupled toutilization circuits 34. Circuits 34 may be for example a weather radarcircuit for detecting and displaying storm intensities corresponding tosignals received at antenna 22 from various points in the atmosphere.

A coupler 40 couples a portion of the signals appearing at terminal 16bto one input terminal 42a of mixer, 42, which may be a single diode typemixer. A coupler 43 couples a portion of the signals appearing at port14a to another input terminal 42b of mixer 42. Mixer 42 produces at itsoutput terminal 42c a frequency which is the difference of thefrequencies applied to terminals 42a and 42b. The difference of IFsignal appearing at terminal 42c is amplified by an amplifier 44 andpassed to a frequency-to-voltage discriminator 46. Discriminator 46produces a voltage which is a function of the frequency produced bymixer 42. Discriminator 46 is coupled to a sample-and-hold circuit (S/H)48. The inverting output terminal of S/H 48 is coupled to the controlterminal of a second oscillator 50. As will be described hereinafter,S/H 48 is of the type which produces for higher input voltages arelatively lower output voltage and produces for relatively lower inputvoltage a relatively higher output voltage. Oscillator 50 is typically avoltage controlled oscillator which responds to the voltage signalsreceived from circuit 48 to produce a corresponding output frequencywhich is applied to switch arm 16c of switch 16 producing an increasingfrequency for increasing control voltage. The voltage produced atcircuit 48 is such that VCO 50 produces a frequency, f₂ of, for example,9345 MHz. Relative to the power of oscillator 12, the power ofoscillator 50 is greater by, for example, a factor of ten.

The position of switch arm 16c is determined by a timing and controlcircuit (T/C) 52 which is also coupled to S/H 48 to determine whethercircuit 48 is sampling the signal produced by discriminator 46 orholding the signal previously sampled. T/C 52 is also coupled toutilization circuit 34 to control the response to signals from amplifier32. T/C 52, which may be a crystal oscillator having a frequencydigitally stepped down to desired values in conventional manner,produces a series of spaced-apart pulses. For example it may produce 200pulses per second, each pulse being of pulse width or pulse timeduration of 10 microseconds.

Switch arm 16c is responsive to the presence of pulses from circuit 52to be positioned to terminal 16b and responsive to the absence of suchpulses to be positioned to terminal 16a. S/H 48 is responsive to thepresence of pulses for sampling signals received from discriminator 46and responsive to the absence of pulses for holding signals so received.Referring momentarily to waveform G, FIG. 3, pulses 60, 62, 64 and 66produced by circuit 52 are illustrated as a function of time. Neitherwaveform G nor any of the other waveforms in FIG. 3 are drawn to scaleeither as to ordinate or abscissa. The waveforms of FIG. 3 are intendedto be used only as an aid in understanding the operation of the circuitin FIG. 1 which will now be described.

The circuit of FIG. 1 is either in the transmit mode or the receivemode. It will be initially assumed to be in the transmit mode.Oscillator 12 is initially assumed to produce a radio frequency signalof given value such as 9342 MHz. Further, it will be assumed that S/H 48produces a voltage such that oscillator 50 produces a frequency f₂ of9345 MHz. It will be additionally assumed that a 10 microsecond pulse(pulse 60, FIG. 3, waveform G, hereinafter, waveform G-60) is presentindicative of a transmitting mode so that arm 16c is positioned toterminal 16b and S/H 48 is sampling voltage from discriminator 46.Therefore, antenna 22 transmits into the atmosphere a 10 microsecondpulse of 9345 MHz as illustrated by waveform H-68. Mixer 42 receives atterminals 42a and 42b signals of frequency 9345 and 9342 MHzrespectively and therefore produces at the terminal 42c an IF signal of3 MHz as illustrated in waveform C-70. Discriminator 46 converts the 3MHz pulse to an appropriate voltage (waveform D-72) which is sampled byS/H 48 under control of T/C 52. The letter "S" at various places alongwaveform E, FIG. 3, indicates times when S/H 48 is supplying voltageproduced by discriminator 46 while the letter "H" identifies times whenS/H 48 is holding a previously sampled voltage.

When the signal supplied at terminals 42a and 42b are 3 MHz apart, S/H48 produces a voltage which causes oscillator 50 to produce a frequency3 MHz above that produced at oscillator 12 or, in the example, 9345 MHz.

During the time pulse 60 (waveform G-60) is present, and therefore an RFsignal is applied at amplifier 18 the RF signal is passed in the reversedirection as indicated by dashed arrow 20a around circulator 20 throughlimiter 24 and amplifier 26 to terminal 30a of mixer 30. Mixer 30 alsoreceives a signal simultaneously from oscillator 12 via circulator 14.Mixer 30 therefore produces an IF signal at terminal 30c. Sinceutilization circuit 34 is disabled by pulse G-60, no action is takenupon the IF signal at this time, At the trailing edge G-60b of pulseG-60 which marks the changeover from transmit mode to receive mode threeevents occur: (1) switch arm 16c is positioned to terminal 16a, (2) S/H48 holds the voltage previously received from discriminator 46, (3)utilization circuits 34 are made active.

When switch arm 16c is connected to terminal 16a, oscillator 12 isconnected to oscillator 50 through circulator 14 as illustrated inFIG. 1. When the two oscillators are connected together, the frequencyof oscillator 50 becomes injection locked to the frequency of oscillator12 such that oscillator 50 produces the same frequency, 9342 MHz, asoscillator 12 and not the frequency which would result from the controlvoltage applied to oscillator 50. Conversely, the frequency ofoscillator 12 is not locked to that of oscillator 50 since oscillator 12is isolated from oscillator 50 by circulator 14. The shift in frequencyfrom 9345 MHz and 9342 MHz occurs in approximately one RF cycle. Thatis, approximately one cycle after arm 16c is positioned to terminal 16ano transmitting frequency signal is present in the circuit of FIG. 1except the signal returned to antenna 22 from the atmosphere. Further,because arm 16c is not connected to terminal 16b no power istransmitting through antenna 22. It will be understood in thoseapplications in which the system design requirements can tolerate atransmitting frequency different in frequency from the frequencytransmitted in the transmit mode, switch 16 can be a single pole singlethrow switch having only arm 16c and terminal 16a. In such anarrangement terminal 16b is connected electrically to arm 16c by meansof an appropriate circulator. With switch 16 as shown no frequency ispresent at terminal 16b during the receive mode and therefore nofrequency is present at terminal 42a of mixer 42. Mixer 42 is thusreceiving 9342 MHz at terminal 42b and being a low pass mixer producesno frequency at terminal 42c. Thus, no voltage is produced bydiscriminator 46. The lack of voltage is of no consequence as S/H 48 isdisabled from sampling input voltage by the lack of a pulse from T/C 52.During the receive mode return signals at 9345 MHz to antenna 22 arerouted by circulator 20 through limiter 24 and amplifier 26 to terminal30a of mixer 30. Mixer 30 is also receptive at terminal 30b of a 9342MHz signal from reference oscillator 50. Mixer 30 thus produces a 3 MHzIF signal which is processed by utilization circuits 34. It will beappreciated that the power requirement of mixer 30 may be greater thanthe power supplied by oscillator 12. However, the power availability ofoscillator 50, frequency locked to oscillator 12, is sufficient tosupply the power needs of mixer 30. It will be noted that in the receivemode the only 3 MHz signal present is that from mixer 30. No 3 MHzsignal is produced by mixer 42 during the receive mode. Such a signalwould be undesirable in that, if coupled to mixer 30 and the circuitstherefollowing, would be treated as undesirable noise.

Upon generation of the next pulse 62 (waveform G-62) by T/C 52, switcharm 16c is reconnected to terminal 16b disconnecting oscillator 12 fromoscillator 50. The control voltage applied from S/H 48 to oscillator 50causes the oscillator to become stable and produce 9345 MHz within oneRF cycle after oscillator 50 is disconnected from oscillator 12. Due tothe presence of pulse 62, mixer 42 continues to produce 3 MHz whichcauses S/H 48 to produce a voltage to maintain oscillator 50 at 9345MHz.

If for some reason oscillator 50 goes up in frequency, mixer 42 willproduce a signal of greater frequency than 3 MHz, the voltage indiscriminator 46 will rise accordingly and the voltage from S/H 48 willdecrease accordingly causing the frequency of VCO 50 to decrease.Conversely, if the frequency of VCO 50 decreases for some reason thefrequency from mixer 42 will also decrease and the voltage from S/H 48will increase causing an increase in the voltage applied to oscillator50 and an increase in its output frequency.

The frequency of reference oscillator 12 may also drift up or down. Thecircuit illustrated in FIG. 1 will follow such drifts. For example, anupward drift of 1 MHz followed by a downward drift of 1 MHz byoscillator 12 is illustrated in FIG. 3. It will be realized that a driftof 1 MHz in one cycle of T/C 52 pulses would not occur in a practicalcircuit but is shown for purposes of illustration only. The 1 MHz driftof oscillator 12 is illustrated as occurring between pulses 62 and 64 ofT/C 52 while a downward drift of 1 MHz back to 9342 MHz is illustratedas occurring between pulses 64 and 66. Therefore when pulse 64 occursoscillator 12 is producing 9343 MHz and oscillator 50 is producing 9345MHz. Mixer 42 thus produces a difference signal of 2 MHz which istranslated by discriminator 46 to a decrease in voltage signal and byS/H 48 to an increase in voltage signal to increase a frequency ofoscillator 50 to 9346 MHz, thus maintaining a 3 MHz separation betweenoscillator 50 and oscillator 12. When pulse 66 occurs oscillator 12 hasreturned to a frequency of 9342 MHz but the voltage produced by S/H 48will initially cause oscillator 50 to produce 9346 MHz resulting in a 4MHz difference signal being produced by mixer 42 with a result indecrease in signal produced by S/H 48 and a decrease in frequency ofoscillator 50 back to 9345 MHz thus oscillator 50 continues to followoscillator 12 in frequency.

It will be noted that mixer 42 and mixer 30 produce essentially the samesignals although their output terminals are connected to differentcircuits. FIG. 2 is a circuit essentially identical to FIG. 1 exceptthat mixer 42, couplers 40 and 43 and amplifier 44 and eliminated. Anadditional switch 90 similar to switch 16 is added. Amplifier 32 isconnected to arm 90c rather than directly to circuit 34 which isconnected to terminal 90a. Terminal 90b is connected to discriminator46. Arm 90c is positioned under the control of T/C 52 to terminal 90awhen arm 16c is connected to the terminal 16a and arm 90c is positionedunder the control of T/C 52 to terminal 90b when switch arm 16c ispositioned at terminal 16b. Thus, as illustrated in FIG. 2, acombination of mixer 30 and switch 90 accomplishes the function ofmixers 30 and 42 in FIG. 1. It will be realized that during the transmitmode (i.e. when arm 16 is positioned to terminal 16b) the power suppliedto terminal 30b of mixer 30 by relatively low power oscillator 17 isconsequently low. However, since the total power gain of amplifiers 18and 26 is sufficient to overcome the isolation of circulator 20 andprovide ample power at terminal 30a of mixer 30 to allow properoperation of the mixer.

What is claimed is:
 1. A pulsed radar oscillator comprising in combination:a first means producing a signal of a given radio frequency f₁ ; a second means normally adapted to produce a signal of a second different radio frequency f₂ but responsive to the application of said signal of frequency f₁ thereto for producing frequency f₁ ; means responsive to said second means for transmitting said frequency f₂ when produced thereby; means producing a succession of time spaced pulses of one value alternating with a succession of time spaced pulses of another value; and means responsive to said pulses of one value for applying said frequency f₁ to said second means and responsive to said pulses of another value for removing said frequency f₁ from said second means.
 2. The combination as set forth in claim 1 further including a subtractive mixer responsive to said frequency f₁ from said first means and responsive to said frequency f₂ for producing the difference frequency thereof, said difference frequency being an intermediate frequency.
 3. The combination as set forth in claim 2 wherein said second means is a voltage controlled oscillator.
 4. The combination as set forth in claim 3 further including means responsive to said intermediate frequency for producing a control voltage which is a function of said intermediate frequency, said control voltage being applied to said voltage controlled oscillator to control the frequency thereof.
 5. The combination as set forth in claim 4 wherein said mixer produces said intermediate frequency only when said second means is producing said frequency f₂ and further including means responsive to said means producing a succession of time spaced pulses and to said control voltage for sampling said control voltage when said pulses of another value are present and for storing said control voltage so sampled when said pulses of one value are present.
 6. The combination of claim 1 wherein said first means is a transferred electron oscillator.
 7. The combination of claim 1 wherein said means for transmitting said frequency f₂ comprises a radio frequency signal amplifier coupled to said second means for amplifying signals therefrom and a transmitting antenna for transmitting said amplified signals into the atmosphere.
 8. The combination of claim 7 wherein said means responsive to said pulses is a switch means.
 9. The combination of claim 8 wherein said switch means couples said first means to said second means only when said pulses of said one value are present and coupling said first means to said radio frequency signal amplifier only when said pulse of said another value are present.
 10. The combination as set forth in claim 7 wherein said transmitting antenna is also a receiving antenna for receiving reflected signals of said frequency f₂ and further including a mixer receptive of said received signals of frequency f₂ and receptive of said frequency f₁ for producing a signal of frequency which is the difference of f₁ and f₂. 